Fig. 1: The CO₂ to CO conversion process on graphene-supported nickel single site.

a, b In all, 4 × 4 periodic Ni–SAC system containing Ni–N₄ and Ni–N₂C₂ moieties as active sites for CO reduction reaction. c The optimized structure of physically adsorbed CO₂ on Ni single site, is stabilized by three molecules of explicit water from solution. d, e The optimized structure of chemically adsorbed cis-COOH (H up) or trans-COOH (H down) respectively. The arrow sign represents the reaction direction where physically adsorbed CO₂ molecule reacts with a neighboring water molecule to produces cis- or trans-COOH intermediates together with hydroxyl ion. The later is stabilized by two explicit water molecules in the solution. f, g Optimized adsorbed cis- and trans-COOH intermediates on Ni–SAC respectively. h, i Optimized structures of CO product binding on Ni–N₄ and Ni–N₂C₂ site respectively. The arrow sign represents the reaction direction of chemisorbed COOH with water and produce CO and hydroxyl ion in solution. The CO binds on the Ni–N₂C₂ site perpendicularly representing a stronger attraction than on the Ni–N₄ site. The whole calculation was done by using implicit solvation as in VASPsol and CANDLE solvation as implemented in the VASP and jDFTx code, respectively together with three explicit H₂O to better describe the charge transfer and polarization. Supplementary Figs. 1 and 2 show that adding additional explicit waters leads to similar results, and supplementary Table 6 shows that reoptimizing the structures with jDFTx leads to negligible changes compared to the optimum structures in VASP. (Gray color to entire surface represents implicit solvation, brown: carbon, blue: nitrogen, green: nickel, red: oxygen, off white: hydrogen atom).